The onset of acute myocardial infarction (AMI) has been reportedly related to weather conditions. The aim of this study was to investigate the impact of weather conditions on AMI onset. Our study population consisted of 274 patients enrolled in the Oita AMI Registry who were admitted with AMI between June 2012 and May 2013. We divided the 365 days of the year into the four seasons: spring (March, April, May), summer (June, July, August), autumn (September, October, November), and winter (December, January, February). We classified each day as a day of onset of AMI (onset day) or a day of non-onset of AMI (non-onset day). Information on maximum temperature, minimum temperature, mean humidity, and mean atmospheric pressure was obtained from the Japan Meteorological Agency. In summer, the temperatures and intraday temperature differences were significantly lower on onset days than on non-onset days. Receiver operating characteristic analysis for predicting AMI onset in each season showed that the maximum temperature 2 days before AMI onset in summer had the largest area under the curve (AUC = 0.72, p = 0.0005). Our analysis demonstrated that there exist specific weather conditions that influence AMI onset in each season in Oita prefecture. AMI onset in summer was particularly associated with the maximum temperature 2 days before AMI onset.